In Britain, the song thrush (Turdus philomelos) is categorized as a species of high national conservation concern because of a large population decline during the last three decades. Song thrushes ringed as nestlings, juveniles and adults during April–September were used to estimate survival rates separately for the post-fledging period, the remainder of the first year and for adults. Daily survival probability was lower during the post-fledging period than during the remainder of the first year or for older birds. Survival of first-year birds is correlated negatively with the duration of the longest run of frost days and the survival of adults is correlated negatively with the duration of the longest summer drought. Variation among blocks in mean PMR is correlated with block means of the duration of runs of frost days and drought days, but significant correlations between PMR and both post-fledging and first-year survival remains after, allowing for the influence of weather on survival. This evidence suggests that changes in survival of birds in their first year of life after fledging have had the greatest impact on population changes for the song thrush species (Robinson et al. 2004).
Birds possess probably the most advanced visual system of any vertebrate and therefore it is natural to expect that the colour of eggs will play a major role in avian life histories in many respects. Brood parasitism is a reproductive strategy adopted by approximately 1% of birds. Obligate brood parasites lay their eggs in the nests of other species (hosts) and these take over the care of incubation and feeding of the parasite nestlings. If successful (i.e. the host accepts the parasitic egg and successfully raises the parasitic chick), then the host breeding success is very low and close to zero. Studies have shown results revealed that two colours originally designed as a mimetic were rejected at a high rate, whereas one group of the nonmimetic was accepted. A multiple regression model of absolute differences between song thrush and experimental eggs on rejection rate shows that the level of mimicry in the UV and green parts of the colour spectrum significantly influenced egg rejection in the song thrush (Honza et al. 2007).
A recent molecular study indicates that the Song Thrush’s closest relatives are the similarly plumaged Mistle Thrush (T. viscivorus) and the Chinese Thrush (T. mupinensis); these three species are early offshoots from the lineage of Turdus thrushes before they diversified and spread across the globe, and hence are less closely related to other European thrush species such as the Blackbird. (T. merula) (Voelker et al. 2007).
Reference List:
- Honza.M., Polacikova.L., & Prochazka.P., (2007). Ultraviolet and green parts of the colour spectrum affect egg rejection in the song thrush (Turdus philomelos). Biological Journal of the Linnean Society, 92, 269-276.
- Robinson.A.R., Green.E.R., Baillie.R.S., Peach.J.W., & Thomson.L.D., (2004). Demographic mechanisms of the population decline of the song thrush (Turdus philomelos) in Britain. Journal of Animal Ecology, 73, 670-682.
- Voelker.G., Rohwer.S., Bowie.R.C.K., & Outlaw.D.C., (2007). Molecular systematics of a speciose, cosmopolitan songbird genus: Defining the limits of, and relationships among, the Turdus thrushes. Molecular Phylogenetics and Evolution, 42, 422-434.
